In many industrial sectors it is necessary to transform reels of web material of one size into spools of a different size, by means of a process of unwinding parent reels, or so-called jumbo reels, and rewinding them into spools with different size characteristics. In certain cases the web material from a single parent reel is unwound and divided into longitudinal strips, each of which is wound onto a helically wound spool. The finished spools obtained in this way are used as semi-finished products to feed production lines for other articles.
Machines that produce spools of helically wound web material from parent reels are sometimes called spooling machines. The web material can, for example, be a non-woven fabric. The helically wound spools that are obtained are used, for example, to feed machines for the production of sanitary towels, diapers and other hygienic and sanitary articles. The web material wound on the parent reels sometimes has a transversal size (corresponding to the axial dimension of the parent reel) 5-15 times the width of the individual longitudinal strips that are obtained by longitudinal cutting of the web material on the parent reels. The individual strips are fed simultaneously to helical winding stations, in each of which a helically wound spool is formed. The winding stations are arranged in line one after the other in a machine direction, defined by the direction of advance of the longitudinal strips obtained by cutting the material on the parent reels. Each strip is fed to the respective winding station along a feed path.
As the web material in a single parent reel is subdivided into a plurality of strips, and as these are helically wound onto the helically wound spools, on which a large quantity of cut material can thus accumulate, the helically wound spool production cycle requires the use of a plurality of parent reels. In other words, if the web material from the parent reels is subdivided into N longitudinal strips, for simultaneous formation of N helically wound spools, in order to form the N helically wound spools a certain number M of parent reels will be required, where M is usually higher than 1, typically between 2 and 10, for example between 2 and 8, in certain cases between 2 and 6.
When a first parent reels finishes, it must be replaced by a second parent reel, and the trailing edge of the first web material coming from the first parent reel must be spliced to the leading edge of the second web material wound on the second parent reel. The splicing phase takes place with the machine stopped, i.e. after having stopped all the rotating members, in particular the helical winding mandrels. The machine is also stopped when the helically wound spools have been completed and must be unloaded from the respective winding mandrels, to be replaced with empty winding cores, upon which a new series of helically wound spools is formed.
As winding of the longitudinal strips takes place in helical turns, the winding mandrels are provided with a rotation movement and a reciprocating translation movement parallel to the rotation axis of the winding mandrel. The feeding speed of the longitudinal strips must be as high as possible to increase the productivity of the machine, but it must take into account the fact that the winding mandrels are subjected to accelerations every time the reciprocating translation movement is reversed. Above all during the initial phase of winding the helically wound spools, when the diameter of the latter is very small, it is not possible to use the maximum feeding speed of the longitudinal strips. This, in fact, would involve reversing the reciprocating translation movement of the helical winding mandrels too frequently, and consequently accelerations and dynamic stress that are too high.
Consequently, at least during the initial phase of winding the helically wound spools, the feeding speed of the individual longitudinal strips, i.e. the linear speed at which the longitudinal strips advance along the individual feed paths, must be kept below the maximum speed achievable by the machine, with a consequent reduction in productivity.
In order to manage the acceleration phase of the feeding movement of the longitudinal strips, empirical expedients are currently used, which are frequently left to the initiative and skill of the technician in charge of the machine. Acceleration is normally carried out in steps, setting a sequential feeding speed, i.e. a linear speed of advance of the longitudinal strips that is kept constant for an interval of time, in order to increase the diameter of the helically wound spools. After a certain interval, considered sufficient to obtain a given increase in the diameter of the spools being formed on the helical winding mandrels, the feeding speed is increased to a higher value, which is then kept constant for a further interval of time, and so on, until reaching the maximum linear feeding speed allowed by the machine, which is maintained until the helically wound spools are completed, or until the parent reel is finished. This manner of proceeding is not ideal from the point of view of making full use of the machine production capacity. Furthermore, it requires an adjustment operation by the operator, who must set the speed steps based on a plurality of production parameters, including for example the thickness of the web material, the width of the strip, the angle of inclination of the helical winding and other values.
Similar problems may also occur when winding non helical spools, i.e. when turns of web material are wound spirally rather than helically. In this case winding takes place only with a rotation movement of the spool, without the reciprocating translation movement. During the initial phase of the winding, when the spool only has a few turns, its diameter is very small. An excessively high feeding speed of the web material or of the strip to be wound causes an excessive angular speed that may induce vibrations in the spool, for example due to the not perfectly cylindrical shape of the spool and/or to imbalance in the mass of the spool itself. Thus, even when there is no reciprocating straight movement component, as in the case of helical winding, there may be problems with excessive dynamic stress if the feeding speed increases too quickly during the winding start phase. Problems with vibrations deriving from excessive angular speed are also seen in helical winding machines and are added to those caused by accelerations in the reciprocating translation movement.
There is therefore a need to optimize the starting cycle for winding of a web material, for example in the form of longitudinal strips, onto a spool, in order to optimize the use of the machine and maximize its production.